1. Field of the Invention
The present invention relates to an image heating apparatus applied to an image forming apparatus such as a copying machine, a printer and the like, and more particularly, it relates to an apparatus for heating an image by induction heating and a coil used in such an apparatus.
2. Related Background Art
Conventionally, in image forming apparatuses of electrophotographic type such as copying machines, printers, facsimiles and the like, there have been proposed various fixing apparatuses (fixing devices) as image heating apparatuses for thermally fixing an unfixed (non-fixed) image (toner image) formed and born on a recording material (for example, a transfer material sheet, a photosensitive paper, an electrostatic recording paper, a printing paper, an OHP sheet and the like) by an appropriate image forming process onto the recording material as a permanently fixed image. Among them, there is an apparatus of induction heating type.
The induction heating apparatus comprises a heating member for generating heat by induction current, and an induction coil (electromagnetic induction heating coil or exciting coil) for generating (by high frequency) magnetic flux to be supplied to the heating member, and the image on the recording material is heated by heat from the heating member.
More specifically, induction magnetic flux is generated by the induction coil, and induction current is generated, by the induction magnetic flux, on an inner surface of a metal core of a fixing roller (heating roller) as the heating member, thereby generating heat required for the fixing by Joule heat due to the induction current. In general, a coil (induction coil) obtained by helically winding a conductive wire is disposed within an inner space of a conductive cylindrical roller as the fixing roller, and, by flowing high frequency current through the coil, eddy current is generated on the fixing roller, thereby directly heating the fixing roller.
However, in such a fixing apparatus of induction heating type, since the high frequency current flows through the induction coil, the current flows along only the surface of the conductive wire due to skin effect. Thus, an electric resistance value in this case greatly differs from a direct current resistance value, with the result that self-heating of the induction coil is generated. In this case, if a heat generating amount is great, the coil is thermally deteriorated, thereby shortening the service life of the coil itself or worsening insulation property of the coil. Particularly, when large current from several A (amperes) to several tens of A flows through the coil, if the resistance value is great, a problem regarding temperature increase due to Joule heat of the coil itself becomes serious, and, when the induction coil is disposed within the inner space of the heating member such as the conductive cylindrical roller, since it is difficult to achieve efficient heat discharge, temperature increase of the coil becomes more serious.
Further, since magnetic core loss of a core (magnetic core) having high permeability and constituting magnetic field generating means by combining with the induction coil tends to be varied with temperature, in the image heating operation, if great temperature increase occurs to increase the magnetic core loss of the core, the heating efficiency will be reduced. If the temperature of the core is equal to or more than Curie temperature, magnetism will disappear, with the result that not only adequate heating cannot be achieved but also great load will act on an exciting circuit for supplying exciting voltage to the induction coil.
As a method for solving this problem, although it is considered that the number of wires (strands) of a litz wire is increased, in such a case, the weight of the apparatus is increased accordingly, and the cost of the induction coil is increased accordingly. Further, it is technically difficult to form an induction coil which can be contained within the small inner space of the heating member.
An object of the present invention is to provide an image heating coil and an image heating apparatus, which can prevent increase in temperature of the coil without increasing the number of the windings of the coil.
Another object of the present invention is to provide an image heating apparatus comprising a heating member and a coil for generating magnetic flux, wherein eddy current is generated in the heating member by the magnetic flux generated by the coil, the heating member is heated by the eddy current, an image on a recording material is heated by the heat from the heating member, the coil is constituted by a litz wire obtained by twisting a plurality of insulation coated conductive wires, current of 5 to 50 Amperes are applied to the coil, and an outer diameter of each insulation coated conductive wire is selected to 0.01 to 0.4 mm.
A further object of the present invention is to provide an image heating coil comprising a plurality of insulation coated conductive wires, wherein the coil is constituted by a litz wire obtained by twisting the plurality of insulation coated conductive wires, current of 5 to 50 Amperes are applied to the coil, and an outer diameter of each insulation coated conductive wire is selected to 0.01 to 0.4 mm.
A still further object of the present invention is to provide an image heating apparatus comprising a heating member and a coil for generating magnetic flux, wherein an eddy current is generated in the heating member by the magnetic flux generated by the coil, the heating member is heated by the eddy current, an image on a recording material is heated by the heat from the heating member, the coil is constituted by a litz wire obtained by twisting a plurality of insulation coated conductive wires, the number of windings of the coil is 4 to 15 (turns), and an outer diameter of each insulation coated conductive wire is selected to 0.01 to 0.4 mm.
A further object of the present invention is to provide an image heating coil comprising a plurality of insulation coated conductive wires, wherein the coil is constituted by a litz wire obtained by twisting the plurality of insulation coated conductive wires, the number of windings of the coil is 4 to 15 (turns), and an outer diameter of each insulation coated conductive wire is selected to 0.01 to 0.4 mm.
The other objects and features of the present invention will be apparent from the following detailed explanation referring to the accompanying drawings.